The present invention relates to an all-wheel drive for a motor vehicle, having a first axle that can be driven continuously by the vehicle engine or its gear shift transmission and an automatically operating limited slip differential system. This differential system brings two vehicle wheels of a second axle into driving connection with the first axle. The differential system has a planetary transmission formed of sun gear wheels, planetary wheels and a planetary carrier that has a drive through ratio which only slightly deviates from 1:1. The system also has a braking device which is applied to the planetary transmission at a high ratio.
In an all-wheel drive such as is described in German Patent Application No. P 35 07 490.6, corresponding to commonly-owned U.S. application Ser. No. 835,381, the above type of limited slip differential system is placed in the power train of the front and the rear axle. It has an intermediate planetary transmission in the direct through drive and a braking device that is engageable with the transmission. In order to keep the required braking moment and the braking device small, the transmission ratio of the gear wheel transmission to the braking device is designed to be significantly higher than the transmission ratio for the direct through drive that deviates slightly from 1:1. Optimally, these characteristics are realized by means of a planetary transmission that is known as a Wolfrom transmission.
The Wolfrom transmission has an inlet sun gear wheel and an outlet sun gear wheel that is disposed axially adjacent the inlet sun gear wheel, is connected with the axle to be driven, and has a slightly smaller number of teeth than the inlet sun gear wheel. It also has planetary wheels which mate with the two sun gear wheels, these planetary wheels being disposed at a planetary carrier. A centrifugal braking device is connected with the planetary carrier that becomes effective as soon as the planetary carrier, when there is a speed difference between the driving front axle and the rear axle, is set into rotation. The braking moment of the centrifugal brake rises progressively with the speed difference to cause a torque transmission and thus an automatic joining of the rear axle. The planetary carrier is stopped only when both axles move 100 synchronously, which is practically never achieved. Therefore, an all-wheel drive that has this limited slip differential system may also be called a continuous all-wheel drive.
An objective of the invention therefore is to provide this type of limited slip differential system in the power train of a motor vehicle in such a way that a compact and space-saving construction is made possible and the additional costs resulting from the installation can be reduced to a minimum.
These and other objectives are achieved by the present invention by providing in an all-wheel drive in which the first axle is continuously driven, a limited slip differential system arranged at a second axle for bringing the vehicle wheels on this second axle into driving connection with the first axle. The limited slip differential has a planetary transmission formed by sun gear wheels, planetary wheels and a planetary carrier that has a through drive ratio which only slightly deviates from 1:1, and a braking device that is applied to the planetary transmission at a high ratio. At least one limited slip differential system is arranged at the second axle in direct driving connection with at least one vehicle wheel. If the limited slip differential system is arranged on the axle that is to be joined, for example, in the case of a front-wheel drive motor vehicle, on the rear axle, it can be integrated into the rear axle housing in such a way that a compact and space-saving drive unit is created.
In a preferred embodiment of the present invention, two limited slip differential systems are arranged at the second vehicle axle that can be joined. These limited slip differential systems, in each case, are adjacent to a vehicle wheel and are driven jointly by a central drive shaft extending in longitudinal direction of the vehicle. Thus, the whole axle is not joined, but rather only that vehicle wheel that with respect to the wheels of the other axle exhibits a speed difference is joined. Both limited slip differential systems are rigidly connected with one another and will also be effective if one wheel of the axle exhibits a speed difference with respect to the other wheel. These two limited slip differential systems render a differential gear superfluous and provide, in addition to the all-wheel limited longitudinal slip, a limited transverse slip for the wheels of the axle on which they are mounted. Since a differential gear is not required, the additional cost with respect to a motor vehicle without all-wheel drive is only slight.
The two limited slip differential systems that are identical in their dimensions are arranged symmetrically with respect to the longitudinal axis of the motor vehicle so that their braking devices are each facing one vehicle wheel. The inlet sun gear wheels of their planetary transmissions are driven by central shafts that are flanged together and, via a bevel gear angle drive, are driven jointly by the drive shaft extending in longitudinal direction of the motor vehicle. The longitudinal shaft and the angle drive are housed in a center housing onto which, on both sides, two identically constructed housing caps are flanged in which the limited slip differential systems are housed. The two housing caps, together with the center housing, form a compact axle housing with smoothly blending outer contours, the outer diameter of which may be smaller than in the case of a conventional axle housing since the bevel gear angle drive has smaller dimensions than the previously required differential gear having the ring gear.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.